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Examples of modeling schemes

 

After you move the components and tools in the workspace main window must be connected in the circuit of the future model. If you place the mouse cursor on the output of any component, then the output will be a point with the crosshair. The appearance of this point for connection of the conclusions of the two components by pressing and releasing the LMB, it is necessary to move the cursor to the conclusion of another component (the tool). Until the touch of the mouse to the conclusion of another component, the line connecting components, has a black color, and after touching and pressing LMB - red. At the same time on a connection line to place the figure numbering circuit conductors with a dialog box appearing in the result double click LMB when cursor is located on is subject to the renumbering of the conductor. If thus to open the context menu, you can also change and color guide, matching it with appearing after clicking LMB on the appropriate item in this menu palette.

The program identifies the wires automatically, but by default, their names and symbols are invisible. For the appearance of the diagram symbols conductors automatically need in the Setup menu to bring up a new window Scheme installation and its field circuits to select the item to Display.

In the same way, you can perform the connection of the components with the conductors. Removal Explorer you need to select it and press the Delete key on the computer. The program Multisim allows you to insert components and tools in conductors without reconnection of their conclusions. It is necessary to place the required component on the conductor and release the LMB.

In the simulation it is often necessary to ground the studied schemes. There are two types of grounding: analog and digital. They are both in the component library Sources. Lack of grounding can in some cases lead to failure of the model.

Digital ground is used to create this bus in digital circuits. Moreover, when the digital ground in the work window, it will automatically be connected with all relevant conclusions of the components, so no need to connect it to the components.

For example, input circuits build the circuit, which can be used to verify Ohm's law for subcircuit DC (fig. 1. 3).

Fig. 1. 3. The scheme of verification of Ohm's law for subcircuit DC

 

The ammeter in the circuit of fig. 1.3 has a very small internal resistance, and the voltmeter is very high. Therefore, used in this scheme of measuring instruments for the parameters of the components indicated in the diagram, not have any noticeable effect on the model behavior. The simulation results confirm the validity of Ohm's law for subcircuit DC.

The correctness of this law for the same circuit sinusoidal current can be confirmed with another instrument ? the oscilloscope (fig. 1. 4). In this scheme measurements are performed using the indirect method, but the accuracy of these measurements is much lower as that of a similar oscillographic methods.



To control the current in the circuit in series with the resistor included 1 kOhm resistor 1 Ohm, much less value and, therefore, slightly changing the operation mode of the circuit. Measuring the voltage drop across this resistor and provides a current signal passing through the circuit.

To reduce the error of measurement of waveform voltage and current are shifted along the time axis so that their maximum values coincide with the vertical lines of a screen grid (fig. 1. 5). The scale on the vertical axis of the channel A of the oscilloscope, as seen in fig. 1. 5, is equal to 10 V/div. Therefore, the amplitude value of the signal voltage will be is 1.7∙10=17 V. peak value of a current signal, and the scale on the same axis of the channel, equal to 20 mV / div. there are expressions of 0,85 ∙ 0,02 = 0,017, And the effective value of current from expression 0,017 / = 0,012 A.

The image of the oscilloscope separately from the model verification of the validity of Ohm's law is obtained by copying the whole diagram through a buffer memory in the Paint program, cut pre-selected oscilloscope occurring in conjunction with the room allocated part of the scheme back to the clipboard and paste changed so the contents of the buffer memory to a file in the text editor.

To improve the accuracy of measurement with an oscilloscope you can not only shifting the image signals, but also using reticles which are located on the sides of its screen. The use of reticles with oscillographic measurement methods described in more detail in the Appendix.

The accuracy of determining the amplitude values of voltage and current with an oscilloscope in Multisim program is further enhanced by performing the grid lines of the oscilloscope, the dashed line with equal length strokes.

When observed on the screen of the virtual oscilloscope signals of the simulated processes, in contrast to the physical oscilloscope not synchronized in time of the investigated voltages or currents and voltages of the scan in a traditional display of the result of its actions. Therefore, when a sufficiently rapidly varying processes to enable visual observation of the test signals, you must click LMB on the button located at the top right of the program window Multisim under the switch of the modeling process.

 

Fig. 1.4. The scheme of verification of Ohm's law for subcircuit sinusoidal current with an oscilloscope

 

Fig. 1. 5. The result of model verification of the correctness of Ohm's law for subcircuit sinusoidal current with an oscilloscope

 


Date: 2018-08-27; view: 59


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